1. Field of the Invention
The present invention relates to a surveillance method for detecting faults and fault locations in an optical fiber line of a nonrepeating optical communication line, bi-directional optical communication line, or optical communication line provided with an optical amplifier repeater, and a method of surveilling the gain and loss and the like of optical circuits including optical amplifiers interposed in the optical fiber line.
2. Description of the Related Art
FIG. 8 illustrates a construction of an optical communication line to which a conventional surveillance method is applied. As shown in this figure, a transmitter terminal equipment 110 is connected to one end of an optical fiber line 101, and a receiver terminal equipment 120 is connected to another end of the optical fiber line 101 so as to communicate between both equipments. A remotely pumped optical amplifier 100 is interposed in the optical fiber line 101. The remotely pumped optical amplifier 100 is constructed so as to cascade an erbium doped fiber (EDF) 103 and an optical isolator 102.
The transmitter terminal equipment 110 is provided with a transmitter 111 for transmission of a light signal modulated by transmission data to the optical fiber line 101 and an optical time domain reflectometer (OTDR) 112 that transmits a light pulse to the optical fiber line 101, measures reflection lights and Rayleigh backscatter lights generated therein, and, based on the measurement, surveys the optical fiber line 101 on the side of the transmitter terminal equipment 110.
The receiver terminal equipment 120 is provided with a pump light source 122 for supplying a pump light to the remotely pumped optical amplifier 100, a wavelength multiplexer 121 for transmitting the pump light from the pump light source 122 to the optical fiber line 101, a receiver 123 for receiving and demodulating the light signal transmitted through the optical fiber line 101, and an OTDR 124 for surveying the optical fiber line 101 on the side of the receiver terminal equipment 120 based on the measurement of the reflection lights and Rayleigh backscatter lights.
The optical isolator 102 disposed before the EDF 103 has an optical amplification function to prevent the deterioration of transfer characteristics of the remotely pumped optical amplifier 100 due to the reflection lights and Rayleigh backscatter lights and the like generated in the optical fiber, and it also functions to confine the direction in which light signals propagate.
Although FIG. 8 shows only one-way (the ascent or descent) of the communication line, a bi-directional optical communication line may be formed using the same construction as in FIG. 8.
The optical fiber line 101 is an optical communication line as described above, and the length of the line is, for example, about 400 km. In this case, the remotely pumped optical amplifier 100 consisting of the EDF 103 and the optical isolator 102 is placed within about 100 km from the receiver terminal equipment 120 in consideration of a propagation loss of pump light propagating through the optical fiber line 101 from the pump light source 122.
As shown in FIG. 9(A) signal intensity attenuates in dependence on the propagation distance of a light signal propagating through the optical fiber line 101. The intensity of the light signal attenuates in proportional to the propagation distance owing to the propagation loss given by the optical fiber line 101. The light signal, however, is optically amplified by the remotely pumped optical amplifier 100 to recover the signal intensity at the position where the remotely pumped optical amplifier 100 is interposed.
The surveillance method in the optical communication line thus constructed will now be described. The OTDR 112 provided in the transmitter terminal equipment 110 transmits a light pulse to the optical fiber line 101, and measures the intensity of weak reflection lights and Rayleigh backscatter lights on the time domain as a portion of the Rayleigh scattering light generated by fluctuation of refractive index in the optical fiber line 101, returning to the input.
Since the measurement dynamic range of the OTDR 112 is about 40 dB at the highest, it is impossible to measure the whole section of the nonrepeating optical communication line over 400 km. That is, if the propagation loss of the optical fiber is 0.2 dB/km, the measurable range of the reflection lights and Rayleigh backscatter lights is about 200 km. The reflection lights and Rayleigh backscatter lights received over 200 km becomes so weak as to be buried under noises and discrimination is almost impossible.
In this manner, the OTDR 112 provided in the transmitter terminal equipment 110 surveys the optical fiber line 101 within about 200 km from the transmitter terminal equipment 110 by measuring reflection lights and Rayleigh backscatter lights.
The OTDR 124 provided in the receiver terminal equipment 120 transmits a light pulse to the optical fiber line 101, and measures the intensity of reflection lights and Rayleigh backscatter lights on the time domain returning to the input. In this case, since the optical isolator 102 is interposed at a position of about 100 km from the receiver terminal equipment 120, the light pulse used for measurement does not propagate over the optical isolator 102; and therefore, the OTDR 124 surveys the optical fiber line 101 only up to the optical isolator 102.
In the conventional optical communication line, the optical fiber line 101 is surveyed as described above, and the range of about 100 km from a position over 200 km from the transmitter terminal equipment 110 to a position where the optical isolator 102 is interposed becomes out of surveillance, as shown in FIG. 9(B).
If the receiver 123 surveys a reception level in order to survey the operation of the EDF 103 of the remotely pumped optical amplifier 100, it is impossible to discriminate whether a fluctuation of the reception level results from the transfer characteristics of the EDF 103 or from the optical fiber line 101.